What is the primary function of the pectoralis major muscle?

　　Under normal circumstances, the main functions of the pectoralis major muscle include assisting respiration, adduction and internal rotation of the shoulder joint, enhancing upper limb strength, maintaining postural stability, and protecting internal structures. A detailed analysis is as follows:

　　1. Assisting Respiration

　　The pectoralis major plays an important auxiliary role during respiration. During inhalation, the pectoralis major contracts, expanding the thoracic cavity and assisting the lungs in drawing in more air. During exhalation, the pectoralis major relaxes, aiding in expelling waste gases from the lungs.

　　2. Shoulder Joint Adduction and Internal Rotation

　　Contraction of the pectoralis major enables adduction and internal rotation of the shoulder joint. Adduction refers to the movement of bringing the arm toward the midline of the body, while internal rotation involves rotating the arm across the chest, such as when hugging or lifting heavy objects.

　　3. Enhancing Upper Limb Strength

　　As one of the primary muscles of the upper limb, the development of the pectoralis major directly influences upper limb strength. Strengthening the pectoralis major through exercise can enhance strength and stability of the upper limbs, improving overall physical fitness and athletic performance.

　　4. Maintaining Postural Stability

　　The pectoralis major also plays an important role in maintaining postural stability. During stability exercises or when maintaining certain postures, such as weightlifting or performing a plank, the pectoralis major helps maintain body balance.

　　5. Protecting Internal Structures

　　The pectoralis major is located on the anterior wall of the thoracic cavity and serves to protect vital organs such as the heart and lungs. When the chest experiences external impact, the pectoralis major provides a certain degree of cushioning, reducing the impact on internal organs.

　　The pectoralis major plays a crucial role in the human body, influencing not only respiration and shoulder joint movement but also affecting upper limb strength, postural stability, and protection of internal structures. In daily life, it is important to focus on exercising and protecting the pectoralis major to maintain overall health and ensure proper bodily function.